A wellbore generally refers to a hole drilled into the earth for the extraction of hydrocarbon-based materials such as, for example, oil and natural gas. Because the term “wellbore” generally includes the open hole or uncased portion of a well, the term “wellbore” typically refers to the space bounded by the wellbore wall—that is, the face of the geological formation that bounds the drilled hole. A wellbore is sometimes referred to as a “borehole.”
A perforation is the communication tunnel created from the casing or liner into the reservoir formation, through which oil or gas is produced. The most common method of perforating uses jet-perforating guns equipped with shaped explosive charges. However, other perforating methods include bullet perforating, abrasive jetting or high-pressure fluid jetting. Perforation density is the number of perforations per linear foot. The term perforation density is used to describe the configuration of perforating guns or the placement of perforations, and is often abbreviated to “spf” (shots per foot). An example would be an 8 spf perforating gun. Perforation penetration is a measure, or indicator, of the length that a useable perforation tunnel extends beyond the casing or liner into the reservoir formation. In most cases, a high penetration is desirable to enable access to that part of the formation that has not been damaged by the drilling or completion processes. Perforation phasing is the radial distribution of successive perforating charges around the gun axis. Perforating gun assemblies are commonly available in 0-, 180-, 120-, 90- and 60-degree phasing. The 0-degree phasing is generally used only in small outside-diameter guns, while 60, 90 and 120 degree phase guns are generally larger but provide more efficient flow characteristics near the wellbore.
A perforating gun is a device used to perforate oil and gas wells in preparation for well production. Such guns typically contain several shaped explosive charges and are available in a range of sizes and configurations. The diameter of the gun used is typically determined by the presence of wellbore restrictions or limitations imposed by the surface equipment. The perforating gun, fitted with shaped charges or bullets, is lowered to the desired depth in a well and fired to create penetrating holes in casing, cement, and formation. Thus, to perforate is to pierce the casing wall and cement of a wellbore to provide holes through which formation fluids may enter or to provide holes in the casing so that materials may be introduced into the annulus between the casing and the wall of the borehole.
Current drilling has focused more on directional drilling. Directional drilling results in the creation of lateral wellbores. Lateral wellbores create many difficulties including difficulties with respect to perforating. It is appreciated that arcuate and lateral portions of a wellbore create specific problems, especially with respect to perforating. Further, the longer the lateral portions of the wellbore, the more difficult it is to achieve effective perforations. Thus, as drilling practices are directed more toward directional drilling, and directional drilling creates more and longer lateral wellbores, the need for effective perforating techniques is greatly increased. The need for effective perforating techniques has long existed and the need increases proportionately with the increase in directional drilling.
There has been a long felt need to perforate accurately and efficiently. The types of charges available have restricted such perforating. The available charges are a restriction to enhancing the performance of the perforation. The characteristics of the perforation have been and continue to be inferior. Particularly, the need for a continuous, normal perforation, free from disruption, has long been sought after, but not achieved. Further, the ability to enhance the performance of the perforation has long eluded the art. Especially, the ability to assist and aid the existing charges in the enhancement of the capacity and forcefulness of the perforation has long been desired.
Current perforating devices adapted during casing installation are also problematic. Such perforating devices require secondary control lines that extend to the surface, and are tedious to install and use. It is long desired to have a “disappearing” perforating gun that is unobtrusive after it has been used.
In addition to the problems currently surrounding conventional perforating techniques, today's perforating practices require a great deal of equipment and manpower. For example, the use of coil tubing to initiate the perforating process is costly, time consuming, laden with the need for manpower, and prone to have safety problems.
Notwithstanding the above challenges in the field of wellbore perforation, a significant problem posed in conventional practice is the safety concerns that surround the transportation of a perforating gun to the wellbore site and the associated delays in assembly preparing the device to be conveyed downhole. In conventional systems, perforating charges and the detonators for activating those charges are not installed in a perforating gun prior to arrival at the surface site of the well because of the risk of accidental detonation during transport. The process of assembling the charges and detonator in the wellbore subassembly at the surface site is time consuming and costly because equipment and well services are often billed according to the amount of time that equipment was used or the services were performed at the well. Delays associated with assembling conventional detonators in conventional perforating guns may therefore be costly. It is long desired, therefore, to have a detonator that allows for faster, more efficient assembly in a perforating gun of a wellbore subassembly at the well site.
Even with detonators that allow for faster, more efficient assembly, a problem still exists with pressure-triggered firing assemblies that must be submitted to an in-situ test pressure that is the same or higher than the pressure that will trigger the firing assembly. Before a wellbore can be perforated, some regulations require that the tubulars in the wellbore be tested in-situ at a pressure that is the same or higher than the pressure that will be used when the well is in operation. A conventional pressure-triggered firing assembly is designed to activate the detonator immediately upon reaching a single predetermined pressure. This predetermined pressure must be the same or lower than the pressure at which the well may be operated. Therefore, a conventional pressure-triggered firing assembly could not be used in the pressure test described above because it would fire at or before the test pressure is reached. A solution to this problem may be to use a firing assembly that is not pressure-triggered or requires a secondary control line that extends to the surface. But, such a firing assembly can be costly because of the expense and time associated with installing wireless signal receivers or running secondary control lines. In addition, if there is a failure in the wireless signal receiver or a flaw in the secondary control lines, the perforating gun cannot be fired and another tool must be run downhole to perforate that portion of the wellbore. Pressure-triggered firing assemblies typically have a lower rate of failure and do not require wireless signal receivers or secondary control lines. Thus, there is a need for a pressure-triggered firing assembly that can be exposed to a test pressure without firing but fire in response to a subsequent pressure that is the same or lower than the test pressure.
The present disclosure includes a multi-pressure firing assembly for detonating charges of a perforating gun, in which the multi-pressure firing assembly includes an exterior casing having a first and a second end and forming an internal chamber. As used herein the terms “uphole” and “downhole” are meant to convey their ordinary meaning in the art. Uphole is meant to describe the orientation in a wellbore in which uphole is the end or direction closest to the terranean surface and downhole is the opposite, the end or direction furthest from the terranean surface. For use in a wellbore, it is understood that the multi-pressure firing assembly has an exterior casing with an open internal chamber and is sized to fit in the wellbore where it is used. The multi-pressure firing assembly also includes an actuating assembly in the internal chamber that includes a firing pin.
The disclosed actuating assembly of the multi-pressure firing assembly can also include a piston valve positioned at least partly within a first end of the internal chamber that is rotatable and slideable within the internal chamber. In the disclosed preferred embodiment, the piston valve also includes a slot pin connected to the piston valve in a way that the pin fits into a slot in the exterior casing of the multi-pressure firing assembly. The piston valve is thus on one end of the actuating assembly and the firing pin is on the opposite end. The firing pin is held in place by a retention detent that can be a shear pin. At least one retention detent holds the firing pin in place until a threshold force “shears” the retention detent or causes it to fail and release the firing pin. The typical firing sequence, then, is that a pressure is applied at the terranean surface and this pressure is transmitted through the fluid in the drill string. The pressure then impinges on the firing pin, which is forced against a detonator to set off the charges. In a preferred embodiment of the disclosed multi-pressure firing assembly, the piston valve is uphole from the firing pin and controls the force that reaches the firing pin. It should be appreciated that the multi-pressure firing assembly can be used in a configuration in which the piston valve is downhole and the firing pin is uphole as long as the firing pin is closer to the detonator than the piston valve. Embodiments of the actuating assembly can include other elements as well, such as a compressible gas-containing chamber between the piston valve and the actuator firing pin or other compressible device.
In certain embodiments, the interaction of the slot pin and slot in the exterior casing acts as a multi-pressure mechanical switch such that a first application of an increase in pressure moves the piston valve in the downhole direction and this motion moves the slot pin along the slot until the slot pin reaches a “click” stop or point. At that point, increased pressure will not move the slot pin further and the force from the pressure is not transmitted past the piston valve to the actuating mechanism or firing pin. The actuating assembly also includes a flexible actuation member or biasing mechanism, such as bellows, a spring or other mechanism known in the art, that biases the piston valve in the uphole direction such that a decrease in pressure moves the slot pin further along the slot to the next click point, which again prevents the force from the pressure being transmitted past the piston valve. The slot in the exterior casing can be configured to include one or more additional click points for those uses in which more than one pressure test or increase of pressure in the wellbore is required prior to actuation. When the wellbore is ready for firing of the perforating gun or other actuation of a device or tool, the next pressure increase event moves the pin to a position in the slot that allows the piston valve to move downhole a sufficient distance to transfer the pressure into the internal chamber, causing the retention detent to fail and to drive the actuator into a detonator or a pressure chamber, for example. It should be appreciated that the multi-pressure mechanical switch can be used in a configuration in which the piston valve moves in the uphole direction with an increase in pressure, as long as the actuating mechanism or firing pin is closer to the detonator or other actuation device than the piston valve.
Certain embodiments can be described as including a slot in which the slot defines a series of curves separated by discrete stop points for a slot pin, configured such that a change in pressure from higher to lower, or from lower to higher causes the slot pin to move toward a terminal end of the slot to the next stop point, and wherein when the pin reaches a selected stop point, increased pressure opens the piston valve and allows the force of the wellbore pressure to be transferred to the firing pin or actuating member, effective to cause a retention detent to shear or fail and release the firing pin or actuating member.
Although the slot has been defined in terms of preferred embodiments as a serpentine shape, or a series of curves separated by stop points, it is understood that other shapes could also be used in the practice of the invention. For example, the slot could include one or more straight lines, as in a “zig zag” configuration, or it could be a triangular or other polygonal shape projected onto a curved casing of the device with a slot pin adapted to move within such a shape. All such shapes known or obvious to one of skill in the art would be encompassed or contemplated by the appended claims.
In certain embodiments the disclosure can be described as a downhole multi-pressure firing assembly for a perforating gun that includes an actuating assembly as described. The multi-pressure firing assembly can also include an external casing forming an internal chamber, with an uphole end in fluid communication with a wellbore, and a piston valve disposed in the internal chamber thereof and a downhole end in communication with a detonator, pressure chamber or another firing assembly of a perforating gun, wherein the uphole portion of the exterior casing has a slot configured to accept a slot pin attached to the piston valve with this interaction acting as a mechanical switch responsive to fluid pressure.
In certain embodiments the firing pin is configured to mechanically strike a detonator such as a blasting cap and set off a series of shaped charges in the firing subassembly through firing a detonation cord connected to the detonator and to the charges, for example. The charges can be configured to fire into the center of the wellbore subassembly, through the exterior wall of the wellbore subassembly or both and in certain embodiments charges are configured in pairs in which one of the pair fires into the center of the wellbore and the other fires out the external wall of the casing. The pair of charges can be connected to the detonation cord and positioned to fire substantially simultaneously, or only the first charge of the pair can be connected to the detonation cord and their proximity to each other can cause the first charge to fire the second charge. By substantially simultaneously it is meant that the pair of charges may not fire at exactly the same instant, but are so close together relative to the detonation cord and relative to the other charges connected to the cord that the detonation of the two charges can be considered as a single explosion.
Another aspect of the disclosure can be described as a method of perforating a geological formation adjacent a wellbore with a downhole perforating gun, in which the method includes conveying a tubular string through a wellbore, said tubular string comprising a perforating gun subassembly, wherein said subassembly comprises a casing comprising an inner wall and an outer wall and a plurality of shaped charges disposed in the space between the inner wall and outer wall, conveying the perforating gun subassembly to a location adjacent the geological formation, applying an increased pressure to the wellbore and holding the pressure effective to perform a pressure test of the wellbore, while preventing the firing pin of a firing assembly from actuating by providing a multi-pressure firing assembly in the perforating gun subassembly, effective to prevent force caused by the pressure from reaching a firing pin; and subsequently applying an additional increased pressure to the wellbore in which the multi-pressure firing assembly does not prevent the increased pressure from reaching the firing pin, effective to detonate the shaped charges. In certain embodiments a wellbore may require more than one pressure test or other high pressure application. The multi-pressure firing assembly, including the mechanical switch firing module can be designed to allow for two or more additional pressure increase events prior to releasing the firing pin. In this way, the firing pin and charges can be in place downhole prior to pressure testing or other pressure increase event and can then be fired without any further action from the terranean surface other than applying an increased pressure to the wellbore.
The above general description and the following detailed description are merely illustrative of the generic apparatus and method, and additional modes, advantages, and particulars will be readily suggested to those skilled in the art without departing from the spirit and scope of the disclosure.